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Trophic relationships amongst animals associated with drifting wrack
Wrack accumulates commonly in surf zones of sandy beaches and can be semi-permanent features. Very few studies have investigated the trophic pathways associated with wrack accumulations in sandy-beach surf-zones, despite their potential importance to nearshore food webs. In this study, we were specifically interested in determining the fish-wrack trophic associations in the nearshore. We sampled macrophytes, macroinvertebrates and fish from drifting wrack at two sites with different macrophyte compositions (i.e. algae versus algae/seagrass mix) in South Australia. We sampled the gut contents of fish and analysed the δ13C and δ15N stable isotope signatures of fish, macroinvertebrates and macrophytes. Using both the stable isotope and diet data we identified that fish are feeding amongst wrack accumulations, but some unexplained trophic pathways suggest that fish are also likely to be foraging over multiple habitats elsewhere for food. In contrast, there was more evidence that grazing macroinvertebrates may be feeding on and around macrophytes within the accumulations, as well as using them as habitat. This study thus established some baseline trophic pathways associated with wrack accumulations in sandy-beach surf-zones. Given the modest evidence for use of wrack as a food source, the lower trophic levels of the food webs identified remain unknown and should be an area for future research.
MF17274 Accepted 23 January 2018
© CSIRO 2018